Fair Value Measurements (Assets and Liabilities Measured on Recurring Basis) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives, net	$ 91,590	$ 18,365	$ 28,711
Provisionally price contracts	(97,823)	(134,741)	(80,325)
Convertible preferred securities	7,404	7,154	7,388
Other assets and liabilities	1,278	4,833	8,493
Total	2,449	(104,389)	(35,733)
Level 1
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives, net	60,331	37,229	35,888
Provisionally price contracts	(48,430)	(76,175)	(48,478)
Convertible preferred securities	0	0	0
Other assets and liabilities	5,772	5,186	8,947
Total	17,673	(33,760)	(3,643)
Level 2
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives, net	31,259	(18,864)	(7,177)
Provisionally price contracts	(49,393)	(58,566)	(31,847)
Convertible preferred securities	0	0	0
Other assets and liabilities	(4,494)	(353)	(454)
Total	(22,628)	(77,783)	(39,478)
Level 3
Assets and liabilities measured at fair value on a recurring basis
Commodity derivatives, net	0	0	0
Provisionally price contracts	0	0	0
Convertible preferred securities	7,404	7,154	7,388
Other assets and liabilities	0	0	0
Total	$ 7,404	$ 7,154	$ 7,388